1. Field of the Invention
The present invention relates to vehicle steering system for setting the ratio of the steered angle of a wheel tire relative to the steering angle of a steering wheel depending on a driving situation without causing any sensation of discomfort to a driver.
2. Prior Art
Conventionally, a technology called steer-by-wire is known which connects a steering wheel to a steerable vehicle wheel via electric signals and utilizes an electric control for improving performance and for securing mountability into vehicles or vehicle cabin space. However, such a device lacks reliability against an electric failure, etc. and has difficulty in transferring a reaction force of a road surface applied to a steering wheel by means of motors; therefore, has not yet been put to practical use. As an alternative vehicle steering system which is functionally equivalent to the above system and therefore can continuously vary the ratio of the steered vehicle wheel angle relative to the steering angle of a steering wheel, a system using mechanical gears is disclosed in Japanese Patent Application Laid-open No. 2000-211541 and is practically used. This type of steering system has a high power steering actuator and are provided with a variable ratio transmission mechanism using gears and motors somewhere in a steering column, in which the motors are controlled to adjust the output/input ratio, and the steering actuator is cooperatively controlled to set the ratio of the steered angle of a steerable vehicle wheel relative to the steering angle of a steering wheel depending on a driving situation. This system still use a steering column, thus reliability is secured and there is no need of virtually creating any reactive torque.
Japanese Unexamined Patent Application Publication No. 2003-276617, etc. disclose a steering system which uses a cable instead of a steering column for securing layout flexibility. This steering system uses a flexible cable for connecting a steering wheel to a rack. Thereby, flexibility in space and layout can be secured. Further, a steering system in known in which a variable steering angle ratio mechanism is combined with such a system as described above to enable the setting of the ratio of the steered angle of a steerable vehicle wheel relative to the steering angle of a steering wheel depending on a driving situation.
Japanese Patent Application Laid-open No. 2001-122140 discloses a steering system in which a steering wheel is connected to a steerable vehicle wheel via a hydraulic line, and pumps and valves are used to realize a variable steering transmission ratio mechanism. Japanese Patent Application Laid-open No. 2005-082007 also discloses a steering wheel which uses hydraulic lines for connection to realize a variable steering transmission ratio mechanism. These systems secure flexibility in space and layout similarly to the above-mentioned systems using cables.    Patent Document 1: Japanese Patent Application Laid-open No. 2000-211541    Patent Document 2: Japanese Patent Application Laid-open No. 2003-276617    Patent Document 3: Japanese Patent Application Laid-open No. 2001-122140    Patent Document 4: Japanese Patent Application Laid-open No. 2005-82007